1. Field of the Invention
The present invention relates generally to transvenous cardiac stimulation leads and more particularly to a transvenous lead having a self-actuating fixation mechanism.
2. Prior Art
Transvenously placed cardiac stimulation leads have been used with implantable pacemakers for several decades. More recently, such leads have become the preferred choice for use with implantable cardioverter/defibrillators ("ICDs" or "defibrillators"). Transvenous leads, also called endocardial leads, are typically implanted through an incision in a vein in the area of a patient's collar bone, such as the subclavian vein, and are then guided through the vein to the superior vena cava (SVC) and then to a location in either the fight atrium (RA) or through the atrium and the tricuspid valve and into the right ventricle (RV). In several of the different possible applications, the lead requires a fixation mechanism to hold the lead in place, at least until natural fibrotic growth occurs which may then serve to secure the lead in place. A particular example of such a lead is a right ventricular defibrillation lead which typically includes a pacing/sensing tip at the distal end of the lead for positioning in the RV apex and a defibrillation electrode which extends along the lead body starting from a point spaced apart from the distal end to a location approximately where the lead passes through the tricuspid valve.
One problem which must be addressed in designing a transvenous lead is the overall profile or outside diameter of the lead body and its various mechanisms. These may include pacing and defibrillation electrodes, joints used to assemble sections of the lead together and fixation mechanisms. The lead profile is important because any protrusions, particularly sharp ones, or extra large sections may cause trauma to the vein during insertion. The introducer sheath and/or vein incision must be sized to allow the largest diameter of the lead to pass through. It is therefore advantageous to provide a lead having a fairly constant and minimized profile.
A number of different fixation mechanisms have been developed to help secure the lead distal end in the RV apex. Citron et al, U.S. Pat. No. 3,902,501, "Endocardial Electrode", which patent is incorporated herein by reference, discloses an endocardial lead having a plurality of pliant tines which extend from the electrode adjacent the tip and form an acute angle with the electrode body. Various means are disclosed for holding the tines against the lead body during insertion while allowing their release when the tip is in position. The released tines then cooperate with the heart tissue, particularly the trabeculae, to maintain the electrode tip in position. In one embodiment, a hold down shroud is positioned near the lead tip and is adapted to receive at least the ends of the tines and restrain them in position. The tines are released by either inflating a balloon underlying the tines, pulling a line attached to the shroud or stretching the end of the lead by forcing a stylet against the tip. A drawback with each of these designs is that they require additional mechanical complexity which may increase the lead profile and/or increase manufacturing complexity.
In U.S. Pat. No. 4,722,353, to Sluetz, "Stabilizer For Implantable Electrode", tines which extend perpendicularly from the lead body are made sufficiently pliable such that they allegedly do not cause trauma when they are folded back by the vein wall upon insertion of the lead into the vein. It is not clear, however, that this design does not generate any trauma or that it is in fact effective in achieving fixation.
U.S. Pat. No. 4,913,164, "Extensible Passive Fixation Mechanism For Lead Assembly Of An Implantable Cardiac Stimulator" to Greene et al, discloses a fixation mechanism which includes a plurality of tines which are movable from a first unextended position to a second extended position using an essentially umbrella-type actuation mechanism. This system thus includes a significantly complex mechanical actuation for the fixation mechanism which would be difficult to manufacture and which could exhibit reliability problems.
In an alternative design which is referred to as an active fixation mechanism, a helical screw tip is rotationally actuated to screw the tip of a lead into the heart tissue. U.S. Pat. No. 4,827,940, to Mayer et al, "Soluble Covering for Cardiac Pacing Electrode", discloses such a screw tip which has been encased in mannitol to cover the sharp protrusions of the fixation mechanism during insertion of the lead. The mannitol dissolves upon exposure to body fluids and then the tip may be secured in the desired location. This design again requires a somewhat complex mechanical design to achieve fixation.
It is therefore an object of the invention to provide a transvenous lead which has a mechanically simple fixation mechanism.
It is another object of the invention to provide a transvenous lead system with a fixation mechanism having a minimal profile during implantation.
It is a further object of the invention to provide a transvenous lead with a fixation mechanism which is relatively simple to manufacture.